Gravity meter



Oct. 21, 1952 R. T. CLOUD ,4

' GRAVITY METER Filed March 5, 1947 2 SHEETS-SHEET l ZHJ/EHTUF' RAYMOND7. C1000 Oct. 21, 1952 R. T. CLOUD 2,614,432

' GRAVITY METER Filed March 5, 1947 2 SHEETS-SHEET 2 AWV'EHTUF PatentedOct. 21, 1952 UNITED STATES PATENT OFFICE GRAVITY METER Raymond TL.cmua; Houston, Tex., assi'gnor to North American Geophysical Company,Hous ton,."lex., a corporation of" Texas 'sppucaemn ns, 1947, screens.mates (01-. 73- -3835) Z1. Claims.

inventionrelates to: a methodand appa' ratus: for effecting: theaccuratercomparison. oi the stress; value in apairt of. identicalcoilsprings,

and has particular application in obtaining av reliable-and. highlyaccuratebalanceof stressesin the-support and reference springs of aninstrulnentv of the. type utilized in measuring the force oi gravity,commonly called a. gravity meteu. I

. The typeof gravity'meter' which heretofore has enjoyed thelargest.degree of use generally involves: a. mass member w-hich issupported byoneor more springs, one-of which. is used to support the bulk of themass, andxw-ill be referred ,to-asthe mainsupport spring. Thegravitational forceacting uponfthe mass member at any" given location.of the instrument'on: the earth's; surface isythen. determined byascertaining the stress produced; in the. suprmrt;springv or morefc'ommonly. byadjustingan'a-uxilia-ry support: spring to.return-thestress conditions in the main supportspring toa previously calibratedvalue.

. Heretofore, I the. ascertainment. of. a predeten mined stressconditionin the.- support spring has been difli'cult of accomplishment with therequired. degree of accuracy. .The extremeaccuracy required is indicatedby the fact. that for oil exploration purposes, itisdesirable to ascertain. variations in the'f'orce' of gravity on the order of 10*. sincetemperature variations of the components of the gravity meter producefactory inasmuch as: the apparatus must necessarily: produce a, visualindicationexteriorly of the insulated container,v and cannot be read atpoints remote from the location of themeter or be.- convenientlyantomatically and continuously recorded; under field conditions.Furthermore; thezmagnificationof the optical system islimited bytherlong physical path. required,- the amount of. lightlavailablai andthe physical. dimensions: of hairlin'es. and; reticules generally"utilized for in.- dicatingpurposes. J t

.fAecordingly. it. is an objector: this; invention to, made. an.improved gravity met'eigs'cliare-c terized. by anoutstanding; simplicityof. the'support. spring: stress indicating. system: and. by an unusuallyhigh. degree. of accuracy by which the attainment of a particularstress], condition in the supportsp'ring may betdetermined.

.Another object- 051 this. invention 1 is to provide animprovedmethodotdeterminingfvariationsiin theforce of gravity.

Another object of this. invention is to. provide an improved gravitymeter wherein. the stress conditions in. asupport-spring. are:determined by comparison. with an identical reference spring. Af-urthezrobject ofi thi invention is to provide an improved method-endapparatus for efiecting a determination. of. the existence of. balancedstress conditions in two: independent but identical coil' springs,particularly the support: am'd. refer! 'ence springszof' a gravitymeten.

I Another object of. this..invention. is toprovi'de an improved stressbalanceindicating. system; for the. support and reference springs of agravity meter wherein. the changes in electrical: vcharlieteristics ofthe: springs:- produced vbythe variation of stressestherein' areutilized. to indicateethe attainment. of; th condition of: balanced.stresses between the-twosprings.

A particular object of this invention is. toprovide an improved.stressbalancing indicating system fora gravity meter wherein allof.the-temperature afi'ected components maybe: conven ientlyencased. withina constant: temperature compartment commonly provided. for such grav itymeters: and whereinthe adjustmentoi. the forcemeasuring spring; may beefiected auto maticallybya; power device which operates in response:tothe departure of' the stress-condition in: the-twospringsifrom abalanced condition,

. The specific natures-of this inventionas w'ellaas other objects andadvantages thereofv will become apparent to those skilled in the artfromthe following" detailed descriptiomot the: annexed sheets. of: drawings,which byway of preferred examples only; illustratetwo-specificembodiments-ofthe invention.

On. the-drawings:

Figure .1- is a schematic view illustrating. an embodiment of thisinvention. based. uponlthe utilization. of an alternating. currentWheatstone bridge as the basic circuit for indicating theattainment-iof; balanced stressed condition.

Figure 2; as. schematic'v w are modified form off'apparatusf'mb dying.th invention. wherein ofiamassoci,

. output:

ated". oscillatoni fell on neacmet' the swings-is revv producedas tnoutput fizg g I guenciessare shown the essential elements of a gravitymeter which are afiected by this invention. Within a heat insulated,constant temperature compart ment, indicated schematically at ID, theprimary elements of a gravity meter are mounted. Such elementsconventionally comprise a mass which is suitably supported by a support,spring I4 in such manner that the force of gravity acting upon the massI2 will result in a change in longi: tudinal stress in the spring I4.For example, the mass I2 may be conveniently mounted upon the end of asupport arm 18 which is pivoted at one end in a suitable'anti-frictionbearing 20. The spring I4 has its lower end suitably secured to anintermediate portion of support arm I8 and its 3 I3 to maintain apredetermined calibrated stress condition in support spring I4. Eachtime that such calibrated stress is produced in support spring I4, theposition of "micrometer screw I3 is an indication of the change in forceof gravity from the calibrated value. fective lever arm of mass I2 withrespect to spring 14" could be varied. In either event means are thusprovided for effecting minute adjustments of "the stress in supportspring I4. It should be understood that the'particular adjusting devicedescribed is merely schematicand representative of any one-of well knownforms of stress adjusting-devices which have heretofore been employed inconnection with the measuring spring of a gravity meter.

In accordance with this invention a reference ":DSLSSBS" through asupport bracket 32 and is threadedl engaged by'an adjusting nut 34 forrelative vertical adjustment. 4

'The support spring I4 and the reference spring 26 are both' preferablyformed from nickel alloy wire'and are carefully wound into highlyaccurate helical coils. It is therefore obvious that the wire in thisform has the electrical characteristics of a long solenoid and theelectrical impedance of" such solenoid is very sensitive to changes in'stress in the spring. Not only does the inductance of the spring changedue to "changes in configuration of the spring as a function of thestressthereon, but, in addition the material from which the spring 'isconstructed 7 has'a res stance which is very. sensitive to changes in;mechanical stress. Hence, a. substantial change in effective electricalimpedances of the Alternativel the ef- Q 4 support coil I4 and referencecoil 26 Will occur as a function of the stresses applied to such coils.

In accordance with this invention, identical electrical connections aremade to the two springs, the electrical characteristics of such springsare incorporated in two separate circuits, and theefiective impedancesof such circuits are compared to determine the attainment of balancedstress conditions between the support coil I4 and'the reference coil 26.In the embodiment of this invention illustrated in Figure 1, thesupport'spring I4 and the reference spring 26 are electrically connectedinto opposite arms of a Wheatstonebridge. The remaining arms of thebridge may conveniently comprise fixed resistors 36 and 38 which arepreferably mounted within the constant temperature compartment toachieve a highly sensitive bridge arrangement substantially independentof ambient temperature variations. While not limited thereto, theresulting bridge circuit is preferably supplied with alternating currentfrom a suitable source 40, for the reason that application of DC.current will produce stray fields which may possibly have sorneerroneouseffects upon the indicating instrument. The condition of'balance or thebridge is indicated by the conventional connection of a sensitive meterA across the opposite corners of the bridge to which the voltage source40 is connected.

In operation, the gravity meter is first balanced in a null position bymeans of auxiliary spring :I I and micrometer screw ISusing optical orsimilar means to determine this position, then a minute adjustment ofthe tension in reference spring 26 is effected by rotating adjusting nut34 to obtain vertical adjustment of the threaded rod 39, and suchadjustmentis continued until the in'dicatingmeter A records zerovoltageacross the diagonal of the electrical bridge. When such conditionis attained, it is, of course, an indication that the electricalimpedance of the calibrating spring 26 is then identical to theelectrical impedance of the measuring spring I4. Hence, as initiallycalibrated, the stress conditions existing in reference spring26 willduplicate to a high'degree of accuracy the stress conditions in themeasuring spring I4. In use, the support spring I4 is always returned tothe calibrated condition of stress by adjusting screw I3 to vary thesupport provided by measuring spring II. The extent of adjustment ofscrew I3, of course, is utilized to indicate the change in force ofgravity. Thus the stress is not permitted to vary appreciably in mainsupport spring I4, minimizing mechanicalhysteresis. It is thereforeapparent that this invention provides a greatly improved method andapparatus for indicating stress conditions in a spring by indicating theattainment of balanced stress conditions between such spring and anidentical spring, such as the support and reference springs of agravitymeter.

In Figure 2, a modification of this invention is illustrated Whichisalso based upon the same fundamental concept of utilizing the change inelectrical characteristics of th support and reference springs in agravity meter to effect an accurate indicationof the attainment ofbalanced stress conditions in such springs. The arrangement in Figure 2has an advantage over that of Figure l in that it does not", requireelectrical connections to be made to either of the two springs and,hence, eliminates the problem of insulating at leastone end of suchsprings and a slesa azccommodating cur-rentifiowithroughilightweightcontacts In the modification. ofsi idem-e1 2;

spring Hi; and i measuring spring ll are; disposed in similararrangement zwith respect toftiiecmeass urine mass 'IZ as: washeretofdradescribed; in

connection with-Figural- Likewise, the vrefer? "adjustingmechanism forthe. measuring. spring l l-l- Rotation-of motor 46 in onesdirectionLwillincrease the :te'nsional stress in' measuring-spring til while rotationin an opposite: directionwill decrease-such stress;

conditionof supportspring-1m Again,;thezangular position of? micrometerdevice 4'4 is utilized to.- indicatechanges' in-t'he. forcewof-Tgravityfrom a calibrating value.

' In: accordance with thisdnvention', pair of v Changes' in'stress ofspring H of courseproduce-adjustment: of the stress identicaloscillatorcircuitsare provided:incorporating respectively oscillatortubes 5 0 and 52-.

The oscillatorv circuit associated with tubes 50 and I 52 arerespectively provided with "frequency 7 determining impedances disposedadjacent the support spring l4" and thereference spring-2:6; "such"frequency determining impedances may conveniently comprisextanle coils54 ahd 56whi0h respectively-surround the support' sprin'g I4 and -thereference spring: 28. Theremaining elements of "the oscillating circuitsareentirelyconin the art that each of the-oscillators 5il and'52 will'-generate an output frequency which is a "function of theimpedancecharacteristics ofit-he particularsprin'g with which it iscoupled. T'o compare suchoutput frequencies; picke-up ventional, and itwill be-apparent to-those -skilled coils-58 and 60 areprovidedfwliicharerespectively coupled with tanli cells 54 an'd 56:; and the frequencyofthe voltages induced iir the pickmp coils-are comparedlto produceacomparison volt- -age' whichis afun'ctionof thedifference.- in suchfrequencies. The comparison of such; voltages may'b'e' accomplished byanyone of 'several well known forms of circuits; such as that commonly;

known as a ring+modulator circuitwhich; as illustrated-in Figure l 2;comprisesa center tapped inductance 62, a center tapped? resistance-64,and

a: bridge arrangementof" lialfF-wave rectifyingvoltage; whose:magnitude. nd. .dire'ctiomisaaiunce tionof: the phase shift of theoutput-frequencies of oscillatorsrfifl" and 52' respectively; Hencaanindicating meter 65,,such asza. sensitive'ammeter connected across the;terminals of: resistor. 64 will accuratelyindicate the: attainment of.zero: fre

quency andphase difierence by a zero. reading.

When uchzero frequency and phase difierence.

existsbetween the frequency generated=by oscilr [.4 and the.referencespring 2.6 areidenticaliand therefore, that the. stressconditions in; such springs. arev likewise substantiallyidenticah.Hence, an. accurate indication of theattaimnnt details of constructionmay, be variedg thrqu'gh of balance-stress conditions: imrcierenmspring.

26 andsupport spring i4 is therehylaittaimds. For additionalclarification-.asstmthln cpfimtion oirzalrreauencyi-comparison:circuimrcfcrcnce may be: made: to? my copcndingzapplication entitled fPenculum; and. Acceleration. (lompensation Method:and? Anparatusi. Seriai. Noz'. 1.81 15; filed Aprilg3g'lfi48t.

' A3 further: ieatu rmofcthcc apparatusrshowmmFigure;2:isxthatzsuclnannaratusz mayrhewonven ientlyrutilizedi ta'efiectcautomaticadiustmenttoi the stress: comziition:v of measurinsprinarl. I an il-.01 corrtmuouslwmaintain atstrcss condition insupport: spring-z ht. identical; .trx that residing in reierencezspring: 26:. To; .efiect: suchflcautoe matic: adj listing. action;thesroltagesfilcrosst can? tor-tapped; resistor: 64 isz appliedzto:thel-grids oi as pain: of; pushepull; conn ct d" amnlifierscifilthrough; a; conventional. anti-hunting: networklllioiresistancesandcondensers..

The. Ioadiimpedance fo the. amplifier tubes 6.8: respectivelycom-priseifield-coiiSlilZ and: at of: motor. 46.. The armature: or. motors-46.1.5

connected. between..- the. central; connection: of

field coilsflland 1 4%.: andzthe-common cathode connection: of;amplifiers; 68. Hence. ion-enema;- larityof voltage appearing.a-crossrcenteretappcd resistor." 64,; the. motor: 46: will be: driven;inane direction and for} theoppositepolaritm.thezmo'r tor; 4.6; willbedriven inthe opposite direction.

The direction of rotation:. ofthe:.motor:.is; 01

I I to maintain-a conditioner.zerocvoltagezacmss center tap resistor6.4. which, of: coursczqcorre sponds: to: attainment. ofbalanced:stress; cohd'i: ticns. in. reference. spring? Z6 and. :maim supportsprin IA,.onattainmentaafzthe:calibnatingzcondh tions;

The: apparatus-a represented; in: Figure: 2 has. aparticularr-advantagein: that: the moton flimsy be: connected to;a:.-continuousrecordingdevice for recording; variations: of the.forceof: gravity overlond .periods;of.=. time atiasihgle 1ocation',.-crmay-he connected .to; some form: of. teiemetering device. suchas::a-Selsyn. motor, wherebyrreadr ings could. be observed.atpointsremcte .from the actual location of the instrument. Shelli-8.11arrangement would be: particularly suitableyfor underwater.- Surveys:wheres the meter; wouldhe loweredzto thebcttom Those; skilledintheartswillv recognize: that in: both; modifications. cf thisz-inrentiomme emu.-

It is therefore apparent that the: method and apparatusembodied in thisinvention cons-ti tute an unusually simple and advantageous-arerangement of readily manufactured components which cooperate to produceadegree vof accuracy and flexibility in operation: of.- a gravity meter"which hasheretofore been unobtainablfi; It will,, of .c.ourse-,. be.understood that vario a wide. range. without. departing. from the'jprinciplesof this. invention...and it is jt'hereforca-not the. purpose. ,to.limit..-

patentggraiiteil. hereon.

otherwise than necessitated by the scope of'the appended claims. sIclaim as my invention: 1 q '1. A gravity/meter comprising a measuringma'sa a main support spring supporting said mass, 'a reference springsubstantially identical to said main support spring, adjustable meansfor varying the stress in the main support spring, afirstelectrical-circuit including. said main support spring as animpedance determining'ele+ merit thereof, asecondelectrical circuitincluding-*said referen'ce spring as an" impedance def -term iningelement thereof, and means for electrically comparing the effectiveimpedance of said'first and second circuits'including an indicatingmeter having a predetermined defiec tionwhen the electrical impedanceefiect of said main support spring equals the electrical impedanceeffect of said reference spring.

2. A gravity meter comprising a measuring inass, a main support springsupporting said mass, a reference spring substantially identical to.said measuring spring, adjustable means for varying the stress of saidmain support spring, and: an electrical bridge circuit including saidmain support springand said reference spring opposed arms thereof forcurrent fiow through said springs, said bridge circuit including meansfor indicating when the electrical impedanceof saidmain support springequals theimpedance or said reference spring.

- '3. A gravity meter comprising a .measuring -mass,- a support springsupporting at least a portion of said mass, areference springsubstantiallyident'ical-to said support spring, adjustable means forvarying the stress in said sup port spring, means defining a constanttemperature'compartment enclosing said support spring and said referencespring, a first electrical cir- 'ouit including said support spring asan impedance determining element thereof, a second electrical circuitincluding said reference spring as an impedance determining elementthereof, and means for electrically comparin the effective impedances ofsaid first and second circuits including an indicating meter having apredetermined defiection when the electrical impedanceeffectsof'said-support spring equals the-electrical-impedance eifects-ofthe said reference spring. I

4. A gravity meter comprising a measuring mass, a main supportspringsupporting said mass, a reference spring substantially identicalto said main support spring, adjustable means for varying the stress insaid support spring, means defining a constant temperature compartmentenclosing said support spring and said reference spring, and anelectrical bridge circuit to said mainsupport spring, adjustable meansfor varying the stress in said main support spring, power means foroperating said adjusta-ble means, a first electrical circuit includingsaid main support spring as an impedance determining element thereof asecond electrical circuit including said reference spring as animpedance determining element thereof, means for electrically comparingthe effective impedance of said first andsecond circuits to produceelectrical energy having a characteristic varying as a function of" thedifference ofthe effective'impedanc'e of said first and second circuits,and means for energizing said power means as a function of saidcharacteristic to operate said adjustable means to continuously equalizethe electrical-impedance effects of said main support spring to that ofsaid reference spring.

'6. A-gravitypmeter comprising a measuring mass, a .main support springsupporting said mass, a reference spring substantially identical to saidmain support spring, adjustable means for varying the stress in saidmain support spring, aifirst oscillator circuit having'a frequencydetermining impedance disposed adjacent said measuring spring, wherebythe output frequency of said first oscillator circuit varies as afunction of the stress in said main support spring, a second oscillatorcircuit having a frequency determining impedance disposed adjacent saidreference springin identical relation thereto as the correspondingimpedance of said first oscillator circuit bears to said main supportspring Wherebythe outputfrequency of said second oscillator circuitvaries as a function of the stress in said reference spring, circuitmeans for comparing said output frequencies of said first and secondoscillator circuits to produce a comparison voltage Variable as afunction of the difference of said output frequencies, and indicatingmeans responsive to said comparison .voltage to indicate the conditionof equal stresses in said main support reference springs. I

7. A gravity meter comprising a measuring mass, a support springsupporting said mass, a reference spring substantially identical to saidsupport spring, adjustable means for varying the stress, in said supportspring, a first oscillator circuit having a frequency determiningimpedance disposed adjacent said support spring, whereby'the outputfrequency of said first oscillatorcircuit variesyas a function of thestress in said support spring-, a second oscillator circuit having afrequency determining impedance disposed adjacent said reference springin identical relation thereto as the corresponding impedance of saidfirst oscillator circuit bears to said support spring, vwhereby theoutput frequency of said second oscillator circuit varies as a functionof the stress in said reference spring, means defining a constanttemperature compartment enclosing said support and reference springs andsaid frequency determining impedances, circuit means for comparing saidoutput frequencies of said first and second oscillator circuits toproduce a comparison voltage variable as a function of the difference ofsaid output frequencies, and indicating means responsive to saidcomparison voltage to indicate the condition of equal stress in saidsupport and reference springs.

8. A gravity meter comprising a measuring mass, a support springsupporting said mass, a

: in;,idcntical zrelation ther t asithesc ilcq sa firsuosciliator;circuit bearsrto said supno :jsn i ia whereby "the: output ;;frecu ncyicf .sas conds il ator ircuit varicszasa u c ion citri -str sinsaidrefercnce; sprin cimuihme n .-'f 1";.' Qm-' naringisaid -:outputfxrequencies of said first and second :oscillator circuits :to producaxcomnar son-voltage variable 5345: a function-pf th difiz re ence :ofsaid -output :frequencies, and: means :icr energizing said xpower nneansas. apfunctionpfsaid-comparison yoltageito operate said adjustable meansto continuously equalize the stress in said supportspring :t'o;that1;in. said reference spring.

9. Apparatus for balancing stresses qin'ttwo atlasessaid=-,cornparisonvoltage to indicate th attainidentical springscomprising meansfforapplying a calibratedstress'to one-of said-springs,means for adjusting the stress rinzthe other of said Y springs, a :firstelectrical circuit. jincludingxsald saidifirst .:and..secondi circuits.including .an indieating meter havinga predetermined deflection whenthe electrical impedance effects of said other spring equals theelectrical impedance effects of -said one-sprin'g;

* lokApparatus for balancing the'stresses intwo springs comprising-meansfor subjectingone of said springs to a calibrating stress, means foradjusting the stress in the other of said springs, an electrical bridgecircuit including said one spring in one arm thereof and said otherspring in an opposed arm for current flow through said springs, saidbridge circuit including means for indicating when the electricalimpedance of said one spring equals the electrical impedance of saidother spring.

11. A gravity meter comprising a measuring mass, a main support springsupporting said mass, adjustable means for varying the stress in saidmain support spring, a first electrical circuit including said mainsupport spring as an impedance determining element thereof, referencemeans having identical impedance determining characteristics as saidmain support spring in one stress condition, a second electrical circuitincluding said reference means as an impedance determining elementthereof, and means for electrically comparing the effective impedancesof said first and second circuits, including an indicating meter havinga predetermined deflection when the electrical impedance effects oftermining impedance disposed adjacent; me n -s in w e by e Q ere of saidfirst oscillator-circuit varies as a a function of the, stress in saidmeasuring; spring, a second oscillatorcircuitfor generatingacalibrating, frequency, circuit means for comparing said outputfrequencies of said first andsecond oscillator circuitsito produce -avcomparisonvolt e variable as ai ncuonr id re c W sai e te ir quencies,and indicating means responsive sto ment one. predetermined stresscondito main supportspring,

seldl l, A; method of v determining the attainment of a predeterminedstress condition in a 5 spring m i newr ridi s va r nc im nc havingidentical electrical impedance determin ing characteristics :as.saidspring in one -stress condition, coupling said spring, and saidrefer ence impedance respectively in identical manner into ,twoelectricalcircuits' to modify :the effective electrieal impedances ofsaid circuit adjusting the stress in said spring, and electricallycomparing the effective electrical' irnpedances of said; circuits toindicate :when the-{electrical irn-v pedance effect ,of said v springandsaid reference impedance are identical.

:15. Amethodfor determining-variations force of gravity-upon ameasuringmass-which comprises supporting at least a portion of themeasuring mass by a support spring, providing-a reference impedancehaving substantially identical electrical impedance characteristics assaid support spring in one stress condition, coupling said spring andsaid reference impedance respectively in identical manner intoelectrical circuits to modify the effective electrical impedances ofsaid circuit, adjusting the stressin said support spring, andelectrically comparing the effective electrical impedances of saidcircuits to indicate when the impedance characteristics of said supportspring are equal to those of said reference impedance.

16. A method of measuring variations in the force of gravity upon ameasuring mass which comprises supporting at least a portion of saidmass by a support spring, providing a reference spring substantiallyidentical to said support said main support spring equals the electricalmass, a main support spring supporting at least a portion of said mass,adjustable means for varying the stress of said main spring, a referenceimpedance having substantially identical electrical impedancecharacteristics as said main support spring in one stress condition, anelectrical bridge circuit including said main support spring and saidreference impedance in opposed arms thereof for current flowtherethrough, said bridge circuit including means for indicating whenthe electrical impedance of said main support spring equals the saidreference impedance.

13. A gravity meter comprising a measuring mass, a main support springsupporting at least a portion of said mass, adjustable means for varyingthe stress in said main support spring, a first oscillator circuithaving a frequency despring, coupling said support spring and saidreference spring respectively in identical manner into two electricalcircuits to modify the effective electrical impedances of said circuits,adjusting the stress in said support spring, and electrically comparingthe effective electrical impedances of said circuits to indicatewhenequal stress is obtained in said support and reference springs.

1'7. A method for determining variations in the force of gravity upon ameasuring mass which comprises supporting at least a portion of themeasuring mass by a support spring, providing a reference springsubstantially identical to said support spring, adjusting the stress inthe reference spring to duplicate that in the support spring for acalibrated condition, and adjusting the stress in the support spring bycomparison with the reference spring to return the stress to thecalibrated value when subject to unknown values of the force of gravity.

18. A gravity meter comprising a measuring mass, a support springsupporting at least a portion of said mass, a reference springsubstantially identical to said support spring, circuit means formeasuring the stress in said springs as a function of a measurableelectrical variable, means for adjusting the stress in said referencespring to equal that in said support spring under a calibrated force ofgravity condition, and means for adjusting the stress in said supportspring under uncalibrated conditions to equal that of said referencespring in response to said measurements.

19. In combination with a null-type gravity meter, a support spring inthe form of a solenoid, an identical reference comparison spring, meansto stress-said support spring in response to variations in the force ofgravity, and electric circuit means connected to said support spring andsaid comparison spring to measure said variations in the force ofgravity as manifested by variations of inductance and'resistance in saidsupport spring from a predetermined null reference value of'saidcomparison spring.

'20. In a gravity meter, the" combination of a measuring mass responseto variations in the force of gravity, a support spring for said mass inform of a solenoid, a reference spring, electric circuit means connectedto said support spring and said reference spring, and means to stresssaid support spring in response to movements of said measuring mass,said electric circuit means including elements for detecting variationsof inductance andresistance in said support spring from a null'valueestablished in said reference spring, whereby said gravity meter may benulled in response to said variations.

V 21. The method of nulling a, gravity meterwhich includes the steps ofstressing and elongating a coil spring in response to variations in theforce of gravity, measuring a null-impedance value in an identicalpre-stressed reference spring, measuring impedance variations in saidfirst mentioned spring by comparison to the null-impedance value of saidreference spring, and nulling said gravity meter in response to saidimpedance variations.

RAYMOND T. CLOUD. REFERENCES CITED The following references are ofrecord in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Resistance Wire Strain GageEquipment for Static and Dynamic Testing," pages 608-613 of ProductEngineering, September 1945.

